The present disclosure relates generally to watermarking technology where a special visual effect is produced by creating different levels of gloss within an image. More particularly, this disclosure relates to a method and system for embedding a gloss watermark into a contone image.
In general, watermarks are used to protect against the copying of a document. Ideally, the content of the watermark is readily observable by a human reader but not reproducible with a copier or scanner. One approach used to produce watermarks is to print an image using clear toner or ink. The clear toner or ink have the effect of producing an image where reflected and diffused light associated with the watermark and detected by an observer changes depending on the angle of view, where the observer can discern the content of the watermark by holding the document at an angle, but the watermark is not detectable by a copier or scanner which is restricted to detecting the image with light reflected at right angles to the document.
Other methods and/or systems to produce watermarks or printed documents which can be read, but not copied, include the following.
U.S. Pat. No. 4,210,346, issued to Mowry et al., and U.S. Pat. No. 5,695,220, issued to Phillips, disclose methods to use a particular white toner and a particular white paper that are designed to have different diffused light characteristics at different angles of view. These systems require specially matched paper and toner.
U.S. Pat. No. 6,108,512, issued to Hanna, discloses a system for producing non-copyable printing in a xerographic printer, where text is printed using clear toner. This system provides a printed document where the optical differences are detectable due to reflectivity differences between the toner and non-toner portions of the printed page. The plastic-clear toner used reflects more light than the paper so a human can read the image by holding the page at an angle where a contrast appears between the lighter appearing toner and the darker appearing paper. However, a copier or scanner is configured to supply light at an oblique angle incident to the printed document and the copier or scanner detects diffused light reflected at a right angle to the document. This system produces diffused light which is approximately equal for both the toned and un-toned surfaces, and the scanner or copier will not be able to copy the original.
Another approach taken to provide copy control for a document includes digital watermarking. As an example, in U.S. Pat. No. 5,734,752, issued to Knox, there is disclosed a method for generating watermarks in a digitally reproducible document which are substantially invisible when viewed including the steps of: (1) producing a first stochastic screen pattern suitable for reproducing a gray image on a document; (2) deriving at least one stochastic screen description that is related to said first pattern; (3) producing a document containing the first stochastic screen; (4) producing a second document containing one or more of the stochastic screens in combination, whereby upon placing the first and second document in superposition relationship to allow viewing of both documents together, correlation between the first stochastic pattern on each document occurs everywhere within the documents where the first screen is used, and correlation does not occur where the area where the derived stochastic screens occur and the image placed therein using the derived stochastic screens becomes visible.
One drawback associated with the watermark methods and system discussed above is the need for special toners/inks and/or paper/substrates, or the requirement of the superimposition of additional prints to allow viewing of the watermark(s).
Other approaches to provide gloss watermarks which do not require these special toner/inks and/or paper/substrates are disclosed in U.S. Pat. No. 7,092,128, issued to Wang et al., and U.S. Pat. No. 7,180,635, issued to Wang et al. Those disclosures provide methods and systems for the manipulation of differential gloss as may be inherent in halftoned images. By selectively applying halftones with anisotropic structure characteristics which are significantly different in orientation, while remaining identical in density, a gloss image may be superimposed within an image without the need for special toners or paper and conventional copiers/scanners will not be able to copy the superimposed gloss image.
This disclosure provides methods and systems to embed texts, patterns, and/or binary images as gloss watermarks into contone (continuous-tone) black and white, or color images, as distinguished from embedding gloss watermarks at the image halftoning stage as disclosed in U.S. Pat. No. 7,092,128 and U.S. Pat. No. 7,180,635.